The present invention relates to a process for thermoforming a thermoplastic resin sheet and an apparatus therefor, and particularly it relates to a process for effectively preparing a variety of formed articles such as a container or the like by heating a thermoplastic resin sheet to thermoform a formed part and a production apparatus thereof. 2. Description of the Related Arts
There have previously been produced formed articles such as containers having a variety of shapes by heating a thermoplastic resin sheet such as an olefinic resin sheet and then thermoforming a formed part with a forming die. As the means for heating the thermoplastic resin sheet in the apparatus for thermoforming, there are known an indirect heating method of heating the sheet by keeping it out of contact with a heating source (Japanese Utility Model Laid-Open Publication No. 134218/1980) and a direct method for heating the sheet by directly placing it in contact with a heating source such as a heating plate or the like (Japanese Utility Model Laid-Open Publication Nos. 99765/1976 and 35962/1978, Japanese Utility Model Publication No. 21399/1988 etc.).
However, according to the aforementioned heating method, corrugation is caused due to the thermal expansion of the sheet during the former half of the heating step which inhibits uniform heating of the sheet, and the drawdown following corrugation is also large. Thus, a special thermoplastic resin sheet having extremely small values of corrugation and drawdown must be used or a sheet having a small width must be used for performing a good forming.
On the other hand, the direct heating method at a heating temperature in the vicinity of and below the sheet melting point often causes wrinkle due to the thermal expansion or drawdown of the sheet or transfer of the heating plate impression, which leads to an inferior appearance of the formed product. Although heating at a temperature below the aforementioned heating temperature causes no wrinkle, impression transfer to, or fusing of the sheet, defective formings are often produced and the heat resistance of the formed product obtained is insufficient unless forming is conducted at an extremely high pressure. Moreover, heating at a temperature higher than the melting point of the sheet causes wrinkle, fusing or impression transfer, and thus special sheets must be used. However, such sheets were successful in the prevention of wrinkle or fusing, but they could not avoid diminishing the gloss of the surface.